Stressed skin safety trenching box

ABSTRACT

A safety trenching box is provided having a &#39;&#39;&#39;&#39;stressed skin&#39;&#39;&#39;&#39; wall construction which permits the box to be strong, yet lightin-weight. Each of the wall members of the box has a pair of end columns which are adapted to be bolted to spreader members of varying widths, so that the box may be used in trenches of differenet size. The wall members are constructed of one or more flexible plates which are curved or &#39;&#39;&#39;&#39;pre-stressed&#39;&#39;&#39;&#39; about a substantially horizontal axis, so that the lateral forces exerted by the sides of the trench are borne by tension and compression stresses in the flexible plates or &#39;&#39;&#39;&#39;skin&#39;&#39;&#39;&#39; of the wall members.

United States Patent Pavese [4 1 Jan. 1,1974

[ STRESSED SKIN SAFETY TRENCHING BOX 221 Filed: May 22,1972

211 Appl. No.: 255,202

Primary ExaminerJacob Shapiro Att0rneyRobert A. Maikis [57] ABSTRACT A safety trenching box is provided having a stressed skin wall construction which permits the box to be strong, yet light-in-weight. Each of the wall members of the box has a pair of end columns which are adapted to be bolted to spreader members of varying widths, so that the box may be used in trenches of differenet size. The wall members are constructed of one or more flexible plates which are curved or prestressed about a substantially horizontal axis, so that the lateral forces exerted by the sides of the trench are borne by tension and compression stresses in the flexible plates or skin of the wall members.

17 Claims, 9 Drawing Figures PATENTEUJAH 1 I974 SHEET 10E 4 PATENTEDJAN 1 1914 3.782.126

saw u or 4 1 STRESSED SKIN SAFETYTRENCHING BOX BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates generally to construction industry safety devices and more particularly to an improved safety trenching box which has a high degree of strength and a lowweight.

2. Description of the Prior Art During the digging of trenches and the performance of other earth excavation work, safety trenching boxes are often employed to prevent the walls of the excavation from collapsing and injuring workmen in the excavated area. For example, when sewer lines are installed, a narrow but fairly deep trench is dug and workmen enter the trench to install the sewer pipe. A safety trenching box or safety drag box is often placed in the trench to protect the workmen from falling earth and stone caused by the collapse of the walls of the trench. Since the earth and stone encountered in construction work may exert very large lateral forces on the sides or walls" of the trenching box, the box is customarily fabricated of steel to provide the high structural strength required. In many of the prior art types of safety trenching boxes, the vertically disposed wall members of the box are formed by a' frame consisting of a number of structural steel or bent plate steel columns and beams which are covered by sheet steel to provide a smooth outer surface in contact withthe walls of the trench. The two wall members are separated by a pair of spreader members which are disposed atthe front andrear of the box to separate the wallmembers a distance corresponding to the desired width of the trench. The spreader members are made in sets of different lengths andare usually boltedto the wallmembers, so that a single safety trenching box may be employed in trenches of different widths.

Because of the high structural strength required, prior art safety trenching boxes have been fabricated of structural steel or bent plate steel members and consequently are very heavy. Since the boxes are often dragged through the trench as the excavation work progresses and are frequently hoisted in and out of the trench by a crane or other piece of construction equipment, it is extremly desirable tomake the boxhave the lowest possible weight. When the safety trenching boxes are manufactured, they must be shipped to the customer and are thereafter frequently transported long distances between jobs. Consequently, it is desirable that the weight of the boxes be reduced as much as possible to minimize shipping and handling charges. Additionally, a suitable safety trenching box should utilize a minimum number of parts to facilitate manufacture and repair. In order to prevent injuries to workmen using the box, the box should be so constructed that earth and stone are prevented from accumulating on the walls of the box, so that such accumulations will not fall on workmen in the box when the box is moved or otherwise disturbed in position.

SUMMARY OF THE INVENTION yet is of compact size and relatively low weight.

It is a further object of this invention to provide a safety trenching box which may be manufactured and repaired with substantial savings in labor and materials.

It is a still further object of this invention to provide a safety trenching box having a strong, light-weight construction which minimizes shipping and handling costs and allows large boxes to be handled with standard construction equipment.

It is another object of this invention to provide a safety trenching box which is so constructed as to minimize accumulations of earth and stone onthe surfaces of the box, to thereby reduce injuries to workmen using the box.

Briefly, the safety trenching box of the invention comprises a pair of earth-retaining wall members and a pair of spreader members connected between the wall members for separating and laterally supporting the wall members against the lateral forces exerted by the sides of the trench. Each wall member has a pair of end columns connected to the spreader members. Face plate means and back plate means are disposed between the end columns of each wall member and the back plate means are connected between the top and bottom edges of the face plate means. At least one of the plate means is given a concavo-convex curvature about a substantially horizontal axis, so that the wall members are prevented from buckling under the lateral forces exerted by the sides of th trench. If desired, tension cables may be connected between the end columns of each wall member to increase the strength of the box for wall members of substantial length.

In one embodiment of the invention, both the face plate means and the back plate means comprise flexible plates having concave-convex curvatures and a doubleconvex frame is disposed between the curvedplates. Another embodiment of the invention utilizes a curved face plate and a substantially coplanar back plate. A third embodiment of the invntion contemplates the use ofacurved face plate and a back plate means comprising aplurality of plano-convex plates connected between the top and bottom edges of the face plate at spaced points along the length thereof. In a fourth embodiment of the invention, the back plate means comprises a flexible back plate having a convex surface facing the center of the trench and a concave surface facing the'side of the trench. The face plate means in this embodiment of the invention comprises a plurality of plano-convex plates connected between the top and bottom edges of the back plate at spaced points along the length thereof. A cover plate is then connected between the end columns and arranged to abut the straight edges of the plano-convex plates.

The nature of the invention and other objects and additional advantages thereof will be more readily understood by those skilled in the art after consideration of the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a perspective view of a safety trenching box constructed in accordance with the teachings of the present invention;

FIG. 2 is a perspective view of the rear portion of the safety trenching box of FIG. I of the drawings;

FIG. 3 is a full sectional view taken along the length of one of the wall members of the safety trenching box shown in FIGS. 1 and 2 of the drawings;

FIG. 4 is a full sectional view of the wall member of FIG. 3 of the drawings taken along the line 4-4 in FIG. 3;

FIG. 5 is an enlarged fragmentary view of a portion of FIG. 4 of the drawings showing the details of the cable sleeves employed in the wall member of the safety trenching box of the invention;

FIG. 6 is an enlarged fragmentary view of a portion of FIG. 3 of the drawings showing the details of the anchoring means employed for the cables used in the wall members of the invention;

FIG. 7 is a perspective view ofa wall member constituting an alternative embodiment of the invention with a portion of the face plate or skin" broken away to reveal details of construction;

FIG. 8 is a perspective view of a wall member constituting another alternative embodiment of the invention; and

FIG. 9 is a perspective view ofa wall member constituting still another alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Referring now to FIG. 1 of the drawings there is shown a safety trenching box constructed in accordance with the teachings of the present invention. The box comprises a first wall member 10, a second wall member 11, a front spreader member 12 and a rear spreader member 13. When the box shown in FIG. 1 of the drawings is lowered into a trench or other excavation, the wall members 10 and 11 prevent the sides of the trench from collapsing and filling the trench with earth and stone. The spreader members 12 and 13 are usually bolted to the wall members so that spreader members of different sizes may be employed to permit the trenching box to be used in trenches of different widths. The box is also provided with a pair of front cutting edges 14 and 15 so that minor outcroppings of earth and stone may be sheared off as the box is dragged through the excavated trench by a piece of construction equipment, such as a crane, for example. The rear spreader member 13 is shown in FIG. 2 of the drawings as being mounted on the wall members 10 and 11 near the top of the trenching box to provide for a suitable clearance between the bottom of the spreader member 13 and the floor of the trench. The size of the clearance depends upon the diameter of the pipe or other equipment to be installed in the trench and the space needed by workmen during the course of construction.

The construction of the front and rear spreader members is not critical provided the members are capable of handling the large lateral forces produced by the pressure of earth and stone on the wall members 10 and 11. In FIG. 2 of the drawings, the rear spreader member 13 is shown as comprising a top plate 16 which is welded to side plates 17 and 18. A truss capable of handling the lateral loads is formed by braces 19, 20, 21, 22, 23, 24 and 25 which are welded together. A steel plate or sheet 26 is welded to the top plate 16, the end plates 17 and 18, and the truss members 19 through 25 to complete the assembly. It will be noted that the truss members 19, 21 and 25 form an arch which permits a substantial clearance to be obtained between the bottom of the spreader member and the floor of the trench. Since the front spreader member 12 differs in construction from the rear spreader member, it will be explained in conjunction with the the following description of the wall members 10 and 11 of the trenching box.

The detailed construction of wall member 11 is shown in FIGS. 3 and 4 of the drawings wherein it is seen that the wall member comprises a front column member 30 and a rear column member 31 to which the front and rear spreader members are respectively bolted. The column members 30 and 31 are of hollow construction and may be formed of welded steel plate or by combinations of steel plate and channel members, for example. The open top and bottom of each column member may be closed by a cover plate 32 to prevent the accumulation of water or dirt in the column. The column members 30 and 31 are connected together by compression plates 33, 34 and 35 and by a top plate member 36 and a bottom plate member 37. In practice, the compression plates 33, 34 and 35 and the top and bottom plates 36 and 37 are usually formed of continuous lengths of plate steel and are welded to the column members 30 and 31 to form a light-weight, but strong frame. It may be noted from an inspection of FIG. 4 of the drawings, that, for reasons which will be explained hereinafter, the center compression plate 34 is of greater width than the compression plates 33 and 35. A face plate 38 of flexible material, such as steel plate or steel sheet, for example, is bent over the surface of the frame and is welded to the top plate 36, the compression members 33, 34 and 35, and the bottom plate 37, so that a convex surface is formed facing the side of the trench and a concave surface is formed facing the center of the trench.

In a similar fashion, a back plate 39 of flexible steel plate or steel sheet is welded to the top plate 36, the compression members 33, 34 and 35, and the bottom plate 37. The back plate 39 forms a convex surface which faces the center of the trench and a concave surface which faces the concave surface of the face plate 38. From an inspection of FIG. 4 of the drawings, it is believed apparent that the cross-sectional area of the wall member 11 has a double-convex shape. A first row of stiffener plates 40 are placed at spaced points along the length of compression member 33 between the facing concave surfaces of the face plate 38 and back plate 39. The stiffeners 40 are welded to the member 33 and the top plate 36 and are suitably contoured to provide a double-convex, cross-sectional area for the frame and wall member. A second row of stiffeners 41 are pro vided between compression plate 35 and bottom plate 37, while a third row of stiffeners 42 are provided between compression plates 33 and 34. Finally, a fourth row of stiffeners 43 are welded in place between members 34 and 35 to complete the frame. The four rows of stiffeners 40, 41, 42 and 43 are suitably contoured to provide the overall double-convex configuration for the frame and the wall member 11. A horizontally disposed steel cap member 44 is welded in place along the top of the wall member to provide additional strength for the welded connection between the top plate 36 and the face and back plates. The cap member 44 also provides a smooth surface for the top of the wall member 11. In a similar fashion, a continuous steel cap member 45 is welded to the bottom of the wall member 11.

The foregoing described construction provides a wall member which is extremely strong but light in weight.

From an inspection of FIG. 4 of the drawings, it may be seen that when a lateral force is exerted on the face plate 38 of the wall member in the direction of the arrow 46, the face plate or skin 38 tends to flex and to resume a coplanar shape. Since the back plate 39 is a flexible plate and is connected to the top and bottom edges of the face plate, it tends to straighten out and to assume a coplanar shape. The compression members 33, 34 and 35 prevent the face plate and back plate from becoming coplanar and are conseqeuntly placed in compression. Accordingly, the back plate 39 is placed in vertical tension so that it functions as a tension plate. Since the face plate and back plate are given concavo-convex curvatures about a substantially horizontal axis, horizontal tension and compression stresses are also set up in these plates which resist the lateral force exerted by the sides of the trench. Since plate or sheet steel has a very high tensile strength and a good compression strength, it is readily apparent that the arrangement disclosed is capable of handling very large loads with a light-weight construction. In prior art types of safety trenching boxes, such as the box shown in U. S. Pat. No. 3,593,528, granted July 20, 1971 to John R. Pavese, for example, the wall members are formed by a substantially coplanar steel face plate which is welded to a very heavy frame formed by structural steel beams and columns, so that the high tensile and compression strength of steel sheet construction is not utilized to advantage in handling'the lateral load imposed on the box. The safety trenching box of the present invention, however, permits the frame of the wall member to be fabricated of relatively thin steel plate and utilizes the high stress point of steel or other suitable materials in the back plate and face plate of the wall member to resist the lateral loading. Accordingly, a light-weight safety trenching box is provided having the required structural strength. It is believed apparent from the foregoing description that the frame member may be easily assembled of light-weight components, thereby minimizing labor and material costs in the construction of the box.

The construction thus far described for the wall members and 11 of the safety trenching box offers a strong, but light-weight, construction for boxes of average length. For trenching boxes of substantial length, however, it may be desirable to provide additional support to prevent buckling of the wall member. To this end, provision is made in the wall member shown in FIGS. 3 and 4 of the drawings for the use of steel reinforcing cables. As illustrated, openings are formed in the stiffener rows 42 and 43 adjacent the compression plate members 33, 34 and 35 and pipe sleeves 47 are disposed therein and welded to the adjacent plate member. When the lateral force 46 exerted by the sides of the trench is applied to the face plate 38, a large horizontal tension stress is created in the back plate 39 in the vicinity of the compression members 33, 34 and 35. Consequently, the tension cables are located adjacent the back plate 39 so that they will bear this tension stress. FIG. 5 of the drawings shows one of the pipe sleeves 47 welded in place adjacent compression member 35. A steel cable 48 is threaded through each of the pipe sleeves 47 on a particular plate member and extends the full length of the wall member between the end columns and 31. The cables 48 may be anchored at each end to the columns 30 and 31 by any convenient means. In the illustrated arrangement, however, wedge anchors 49 are utilized to make s secure connection which will permit the cables to be placed in tension during assembly of the wall member.

One of the wedge anchors for end column 31 is shown in enlarged view in FIG. 6 of the drawings. As seen in FIG. 6, a pipe sleeve 50 is disposed within the hollow end column 311 to provide additional strength for the column at the point where the cable passes through the column. The cable 48 is threaded through the pipe sleeve 50 to the outside of the end column 31 and a cylindrical collar 51 is placed over the free end of the cable. A conical wedge 52 is then placed over the free end of the cable and inserted between the cable and the collar 51, so that as the wedge 52 is hammered or driven into place, the pressure of the collar 51 against the wedge causes the cable to be firmly clamped in place. Accordingly, as the tension on the cable 48 is increased, the wedge 52 tends to be pulled further into the collar 51, thereby increasing the clamping force on the cable. While the disclosed arrangement may be used for both ends of each of the cables in a wall member, in some situations it may be convenient to weld or clamp button heads or mushroom heads to one end of each of the cables and to employ the wedge anchor arrangement on the other end. Regardless of the anchoring method employed, however, when the wall members of long boxes are assembled, the cables are placed in tension, so that they serve to bear a portion of the horizontal stress created in the back plate by the lateral force exerted by the sides of the trench.

Referring again to FIG. 1 of the drawings, it will be noted that the front vertical cutting edge 14 of the box is mounted on the hollow end column 30. In practice, the cutting edge 14 may be formed by steel plates which are welded together to provide an extension of triangular cross-section, so that a sharp, vertical edge is formed on the front of the wall member. In order to provide additional support for the front cutting edges 14 and 15, the plates forming each cutting edge are provided with a horizontally extending flange 53 to which is bolted support members or struts 54 and 55. The struts 54 are welded at one end thereof to the flange 56 of an I beam forming the front spreader member 12, while the strut is bolted between the flanges 53 on the front cutting edges 14 and 15. By virtue of this arrangement, the front spreader member 12 not only serves to separate and support the wall member 10 and 11, but also serves to support the front cutting edges 14 and 15. This permits the wall members to remain substantially parallel while the trenching box is dragged through a trench by a crane or other piece of construction equipment, so that the cutting edges 14 and 15 shear off projecting earth and stone on the walls of the trench. Again, the front spreader member 12 may be made in different sizes to permit the trenching box to be used in trenches of different widths. Both the front spreader member 12 and the rear spreader member 13 are bolted to the hollow end columns 30 and 31 on the walls of the box to permit easy installation and removal.

Referring now to FIG. 7 of the drawings, there is shown an alternative embodiment of the invention wherein the face plate is curved and the back plate means takes the form of a coplanar plate. In this arrangement, a substantially flat or coplanar steel plate 60 is welded between the top and bottom edges of a face plate 61. A second back plate 62 is also welded between the top and bottom edges of the plate 60. The face plate 61 is given a convex curvature facing the side of the trench and the back plate 62 is given a convex curvature facing the center of the trench, so that a lateral force on either the face plate 61 or back plate 62 is converted into a vertical tension stress in the plate 60. Again, this arrangement results in the doubleconvex, cross-sectional area utilized in the wall member of FIGS. 1 through 6 of the drawings. A top cap member 63 and a bottom cap member 64 are respectively welded in place on the top and bottom edges of the wall member thus formed to provide added strength for the welded joints and a smooth top and bottom surface. When a lateral force is exerted on the face plate or skin 61 of the wall member, the face plate tends to straighten out and thereby places the flat plate 60 in horizontal and vertical tension, so that this plate bears the lateral thrust exerted by the walls of the trench. In a similar fashion, a lateral thrust exerted against the second back plate 62 of the wall member also causes the plate 60 to be placed in tension. This construction again utilizes the high tensile strength of steel and similar materials to permit the wall to take very large loads with a ligh-weight construction. Hollow end columns (not shown), similar to columns 30 and 31 in the embodiment of FIGS. 1 through 6 of the drawings, are employed to terminate the wall member shown in FIG. 7 of the drawings and provide anchoring points for the spreader members. If desired, prestressed tension cables may be connected between the end columns within the envelope formed by the face plate and back plate to provide additional strength for boxes of long length. In order to bear the maximum horizontal tension stress, the cables should be disposed adjacent the second back plate 62 or the flat plate 60 if the second back plate is eliminated.

Another alternative embodiment of the invention is shown in FIG. 8 of the drawings. In this arrangement, the back plate means comprises a plurality of planoconvex plates 70 which are disposed in a channelchaped member 71. A face plate 72 of flexible material, such as steel plate or sheet steel, for example, is welded along the convex edges of the plates 70 and disposed within the channel-shaped member 71. Accordingly, the plurality of plano-convex plates 70 are connected between the top and bottom edges of the face plate 72 at predetermined spaced points along the length thereof. A horizontally disposed cutting edge 73 is formed on the bottom of the wall member by means of welded steel plates 74 and 75. Finally, tension cables 76 are passed through the plates 70 adjacent the straighr or plano edge of the plates to provide for extra support if required. The tension cables 76 are anchored in end columns (not shown) similar to the columns 30 and 31 in FIGS. 1 through 6 of the drawings. The same end columns may, of course, serve as the mounting members for the spreader members of the trenching box.

This embodiment of the invention operates in a manner similar to the embodiment shown in FIGS. 1 through 6 of the drawings and the embodiment shown in FIG. 7 of the drawings in that the straight edge 77 of each of the plates 70 is placed in tension when a lateral force is applied to the face plate 72 of the wall member. The lateral force exerted by the walls of the trench on the outer surface 78 of the channel member 71 causes the curved face plate 72 to flex in a direction tending to reduce its curvature, so that the straight edge 77 of the plates are placed in tension to bear the load. Since the face plate 72 is given a concave-convex curvature, it will bear the horizontal tension stresses in the curved portions of the plate adjacent the top and bottom edges thereof. The tension cables 76 may be employed in boxes of long length to bear this horizontal tension stress. Although the channel-shaped member 71 could be eliminated from the disclosed construction, it does provide mounting support and stability for the entire assembly.

Referring now to FIG. 9 of the drawings, there is shown a wall member for a safety trenching box constituting another alternative embodiment of the invention. In this embodiment of the invention, the face plate means comprises a plurality of plano-convex plates 80 which are connected by means such as welding to spaced points along the top and bottom edges of a curved back plate 81. A cover plate 82 is welded to the straight or plano edges of the plates 80 and is adapted to receive the lateral forces exerted by the sides of the trench. A bent plate or angle-shaped member 83 has its edges welded to the top edge of the back plate 81 and to the convex surface of the back plate to provide added strength for the wall member. Tension cables 84 are disposed in the wall member adjacent the convex edge of the plates 80 and are anchored to end columns (not shown). The end columns, as in the embodiments of FIGS. 1-6, FIG. 7 and FIG. 8 of the drawings may be conveniently formed of bent plate steel. The plates 80, the back plate 81, the cover plate 82 and the angle-shaped member 83 may be fabricated of steel plate. The plates 81 and 82 and the member 83 are connected to the end columns (not shown) by means such as welding, so that the wall member can be connected to the spreader members of the trenching box.

In this embodiment of the invention, when a lateral force is exerted on cover plate 82 by the sides of the trench, the back plate 81 is placed in horizontal tension in the vicinity of the tension cables 84. The areas of the back plate near its top and bottom edges are placed in horizontal compression, so that a stable structure is produced. For wall members of relatively short length, the angle-shaped member 83 and the tension cables 84 may be omitted since these elements serve to assist the back plate 81 in bearing the horizontal stresses produced by the lateral force exerted by the walls of the trench. It will be noted that the bottom edge 85 of the wall member provides a horizontally disposed cutting edge which may be used to shear-off projections of earth and stone on the trench walls as the box is lowered or driven into place in the trench. If desired, however, the cover plate 82 need not extend the full height of the wall member, so that an open space is left between the bottom edge of the cover plate and the bottom edge 85 of the wall member to prevent earth, stone, water and other matter from accumulating in the hollow wall member. This embodiment of the invention also provides a strong, light-weight wall member which may be easily fabricated from steel plate or other materials having satisfactory stress characteristics.

It is believed apparent that many changes could be made in the construction and described uses of the foregoing safety trenching box and many seemingly different embodiments of the invention could be constructed without departing from the scope thereof. For

example, it is believed obvious that other frame configurations could be employed to produce the stressed skin feature of the invention. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A safety trenching box adapted to be disposed in an excavated trench or the like to prevent the sides of the trench from collapsing comprising a pair of spreader members; and

a pair of earth-retaining wall members, each of said wall members having a pair of end columns connected to said spreader members,

face plate means disposed between said end columns adjacent the sides of the trench, and

back plate means disposed between said end columns adjacent the center of the trench, said back plate means being connected between the top and bottom edges of said face plate means,

at least one of said plate means being continuously curved with a concavo-convex curvature about a substantially horizontal axis, to prevent the wall member from buckling under laterally applied loads.

2. A safety trenching box as claimed in claim 1 wherein said face plate means comprises a flexible face plate curved to form a convex surface facing the sides of the trench and a concave surface facing the center of the trench.

3. A safety trenching box as claimed in claim 2 wherein,

said back plate means comprises a flexible back plate curved to form a convex surface facing the center of the trench and a concave surface facing the concave surface of said face plate, and

compression plate members are disposed between the concave surfaces of said face plate and said back plate and connected between the end columns of the wall member.

4. A safety trenching box as claimed in claim 3 wherein tension cables are connected between the end columns of each wall member adjacent said back plate.

5. A safety trenching box as claimed in claim 3 wherein stiffener plates are disposed between the concave surfaces of said face plate and said back plate and are connected to said compression plate members to form a double convex frame for' supporting the face plate and the back plate.

6. A safety trenching box as claimed in claim 5 wherein a top plate is disposed between the top edges of said face plate and said back plate and is connected between the end columns of each wall member, and

a bottom plate is disposed between the bottom edges of said face plate and said back plate and is connected between the end columns of each wall member.

7. A safety trenching box as claimed in claim 6 wherein the top edges of said face plate, top plate and back plate are connected together and disposed within a first horizontally extending cap member, and the bottom edges of said face plate, bottom plate and back plate are connected together and disposed within a second horizontally extending cap member. 8. A safety trenching box as claimed in claim 2 wherein said back plate means comprises a substantially coplanar plate connected between the end columns of each wall member. 9. A safety trenching box as claimed in claim 8 wherein a second back plate is connected between the end columns of each wall member, said second back plate being connected between the top and bottom edges of said coplanar plate adjacent the center of the trench and being curved to form a convex surface facing the center of the trench and a concave surface facing said coplanar plate. 10. A safety trenching box as claimed in claim 8 wherein tension cables are connected between the end columns of each wall member adjacent said coplanar plate. 11. A safety trenching box as claimed in claim 2 wherein said back plate means comprises a plurality of planoconvex plates connected between the top and bottom edges of said face plate at spaced points along the length thereof. 12. A safety trenching box as claimed in claim 11 wherein said face plate and said plano-convex plates are disposed within a channel-shaped member with the convex surface of the face plate abutting the web of the channel-shaped member and the top and bottom edges of the face plate connected to the flanges of the channel-shaped member. 13. A safety trenching box as claimed in claim 11 wherein tension cables are connected between the end columns of each wall member adjacent the straight edges of said plane-convex plates. 14. A safety trenching box as claimed in claim 1 wherein said back plate means comprises a flexible back plate connected between the end columns of each wall member and curved to form a concave surface facing the side of the trench and a convex surface facing the center of the trench, and said face plate means comprises a plurality of planoconvex plates connected between the top and bottom edges of said back plate at spaced points along the length thereof. 15. A safety trenching box as claimed inclaim 14 wherein a cover plate is connected between the end columns of each wall member abutting the straight edges of said plano-convex plates. 16. A safety trenching box as claimed in claim 15 wherein an angle-shaped member is connected between the end columns of each wall member, said angleshaped member having the edges thereof con- 1 1 l2 nected to the top edge and the convex surface of tension cables are connected between the end colsaid back plate. umns of each wall member adjacent the convex 17. A safety trenching box as claimed in claim 15 edges of said piano-convex plates. wherein 

1. A safety trenching box adapted to be disposed in an excavated trench or the like to prevent the sides of the trench from collapsing comprising a pair of spreader members; and a pair of earth-retaining wall members, each of said wall members having a pair of end columns connected to said spreader members, face plate means disposed between said end columns adjacent the sides of the trench, and back plate means disposed between said end columns adjacent the center of the trench, said back plate means being connected between the top and bottom edges of said face plate means, at least one of said plate means being continuously curved with a concavo-convex curvature about a substantially horizontal axis, to prevent the wall member from buckling under laterally applied loads.
 2. A safety trenching box as claimed in claim 1 wherein said face plate means comprises a flexible face plate curved to form a convex surface facing the sides of the trench and a concave surface facing the center of the trench.
 3. A safety trenching box as claimed in claim 2 wherein said back plate means comprises a flexible back plate curved to form a convex surface facing the center of the trench and a concave surface facing the concave surface of said face plate, and compression plate members are disposed between the concave surfaces of said face plate and said back plate and connected between the end columns of the wall member.
 4. A safety trenching box as claimed in claim 3 wherein tension cables are connected between the end columns of each wall member adjacent said back plate.
 5. A safety trenching box as claimed in claim 3 wherein stiffener plates are disposed between the concave surfaces of said face plate and said back plate and are connected to said compression plate members to form a double convex frame for supporting the face plate and the back plate.
 6. A safety trenching box as claimed in claim 5 wherein a top plate is disposed between the top edges of said face plate and said back plate and is connected between the end columns of each wall member, and a bottom plate is disposed between the bottom edges of said face plate and said back plate and is connected between the end columns of each wall member.
 7. A safety trenching box as claimed in claim 6 wherein the top edges of said face plate, top plate and back plate are connected together and disposed within a first horizontally extending cap member, and the bottom edges of said face plate, bottom plate and back plate are connected together and disposed within a second horizontally extending cap member.
 8. A safety trenching box as claimed in claim 2 wherein said back plate means comprises a substantially coplanar plate connected between the end columns of each wall member.
 9. A safety trenching box as claimed in claim 8 wherein a second back plate is connected between the end columns of each wall member, said second bacK plate being connected between the top and bottom edges of said coplanar plate adjacent the center of the trench and being curved to form a convex surface facing the center of the trench and a concave surface facing said coplanar plate.
 10. A safety trenching box as claimed in claim 8 wherein tension cables are connected between the end columns of each wall member adjacent said coplanar plate.
 11. A safety trenching box as claimed in claim 2 wherein said back plate means comprises a plurality of plano-convex plates connected between the top and bottom edges of said face plate at spaced points along the length thereof.
 12. A safety trenching box as claimed in claim 11 wherein said face plate and said plano-convex plates are disposed within a channel-shaped member with the convex surface of the face plate abutting the web of the channel-shaped member and the top and bottom edges of the face plate connected to the flanges of the channel-shaped member.
 13. A safety trenching box as claimed in claim 11 wherein tension cables are connected between the end columns of each wall member adjacent the straight edges of said plano-convex plates.
 14. A safety trenching box as claimed in claim 1 wherein said back plate means comprises a flexible back plate connected between the end columns of each wall member and curved to form a concave surface facing the side of the trench and a convex surface facing the center of the trench, and said face plate means comprises a plurality of plano-convex plates connected between the top and bottom edges of said back plate at spaced points along the length thereof.
 15. A safety trenching box as claimed in claim 14 wherein a cover plate is connected between the end columns of each wall member abutting the straight edges of said plano-convex plates.
 16. A safety trenching box as claimed in claim 15 wherein an angle-shaped member is connected between the end columns of each wall member, said angle-shaped member having the edges thereof connected to the top edge and the convex surface of said back plate.
 17. A safety trenching box as claimed in claim 15 wherein tension cables are connected between the end columns of each wall member adjacent the convex edges of said plano-convex plates. 